Let's say an alien mothership is cloaked high in the earth's atmosphere. It's not detectable by means such as radar or any usual means of detection aside from crashing into it which is highly unlikely. So how would you detect it? I had an idea of searching for vacuums in the atmosphere standing to reason that no gases would be detectable inside the mothership but is this a workable idea or not.

Edit by usual means I mean radar, heat signatures and spotter planes basically anything that is usually used to detect planes

  • $\begingroup$ If someone has to look for it, you may be SOL. Someone might notice gravitational anomalies, though. $\endgroup$
    – Matthew
    Commented Apr 30, 2020 at 21:34
  • $\begingroup$ So you want to know how to detect something that's undetectable? $\endgroup$
    – sphennings
    Commented Apr 30, 2020 at 23:19
  • 4
    $\begingroup$ You want science based but you're not spelling out what you mean by "usual means" nor what the cloaking technology masks. It's hard to speculate on alternate approaches given both deficits. $\endgroup$
    – user53931
    Commented May 1, 2020 at 13:15
  • 1
    $\begingroup$ How on Earth is this "opinion-based"? The question maybe needs a little more detail to be properly answerable, but closing it seems unnecessarily non-constructive. $\endgroup$
    – Doktor J
    Commented May 1, 2020 at 16:56
  • 2
    $\begingroup$ Especially closing it only 19 hours from its original posting just seems rude. Sure it needs more clarification and constraints... but some people don't stare at their computers all day waiting for input from SE, and might only visit once a day. OP may not have even been back here yet since he originally posted. "On-hold" seems like it would be much more suitable than "Closed"... $\endgroup$
    – Doktor J
    Commented May 1, 2020 at 17:08

7 Answers 7


/It's not detectable by means such as radar or any usual means of detection aside from crashing into it.../

Crash into it, by proxy.

A new radar system from the company LeoLabs is expected to track an estimated 250,000 dangerous objects smaller than 10 centimeters (4 inches) wide that orbit Earth. It's the first commercial device to track debris that small, though it joins a larger radar network that LeoLabs runs to provide real-time data about objects in low-Earth orbit. (That's the zone where most human-made space objects are clustered.)


All the space junk is being tracked. Round and round and round it goes. It is easy to see these angular pieces of scrap metal with radar.

Except at that one spot. Maybe technical trouble? Bring D and J to bear on that spot to watch the junk pass by. Nope, junk disappears there. Usually comes back out the other side but a couple of times, junk gone for good and not even a flash. Hmm... What happens if we send a laser pulse through that area?

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    $\begingroup$ I like this! It's plausible without requiring humans to first know to be looking for something. Even if the ship is too low, maybe there are imperfections in their cloaking that cause anomalies whenever junk passes too closely. $\endgroup$
    – Matthew
    Commented May 1, 2020 at 2:25
  • $\begingroup$ @Matthew if its that low, then you may have birds, jets, or the like "disappearing" into it. $\endgroup$
    – Doktor J
    Commented May 1, 2020 at 16:36
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    $\begingroup$ @DoktorJ, true; I don't actually know offhand how much of a "dead zone" there is between regular flights and "space" junk. I still wonder how much junk is at only 50,000 feet, though. (At which point, saying it's in "space" is a bit of a stretch.) $\endgroup$
    – Matthew
    Commented May 1, 2020 at 17:05
  • $\begingroup$ Space junk that comes below a few 100 km de-orbits quickly, so there is little of it, unless a low altitude satellite recently exploded. Long lived space junk has to stay several 100 to 1000 km up. Commercial aircraft don't go higher than 15 km, and purpose built aircraft don't go beyond ±30 km, so there is quite a gap where no objects come regularly. $\endgroup$
    – JanKanis
    Commented May 2, 2020 at 12:24

"Well, the thing's gotta have a tailpipe."

It is impossible to make a ship that does not release energy in some way; it simply cannot be done.

The alien ship, or more exactly its emissions, are detected by a high-atmosphere probe studying atmospheric phenomena.

  • 3
    $\begingroup$ +1. For additional food for thoughts, what if the alien ship's cloaking technology already handles this by somehow dumping the energy "into another universe"? $\endgroup$
    – justhalf
    Commented May 1, 2020 at 5:46
  • $\begingroup$ @justhalf They still have to use attitude thrusters. $\endgroup$ Commented May 1, 2020 at 11:34
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    $\begingroup$ I'm not convinced about the necessity of tailpipes. Helium balloons don't need them. In fluids you can manoeuver by wiggling your tail like fish. Raise and lower your height using your "swim" bladder. But there should be some heat signature against the night sky... $\endgroup$
    – Jens
    Commented May 1, 2020 at 14:28
  • 1
    $\begingroup$ Any kind of energy conversion releases some heat, but that could be absorbed with a supply of ice carried in the ship. Theoretically a ship can be without heat signature for a limited time, though when you need to re-freeze the molten ice you will still need to get rid of the waste heat at that time. $\endgroup$
    – JanKanis
    Commented May 2, 2020 at 12:08

There are multiple vectors by which the ship may reveal its presence.

Electromagnetic radiation

In particular, the cloaking device may not work in one band of the EM spectrum. In fact, it might glow in that band, releasing all its energy in those frequencies.

Alternately, the cloaking device may create a shadow in the higher energy parts of the spectrum. Consider cosmic rays - the ship's unobtanium hull may block those, and the cloaking device may not affect them, causing a shadow.


The ship may be releasing or ejecting some sort of particles that can be detected. Even if it's just air that was warmed moving past the ship, it can be noticed.

Other Emissions

Parts of the ship - or dust on the hull - could be emitting some sort of radiation (such as alpha or beta radiation, which would be a particle and not affected by the cloak.

Parasite Craft

Parasite craft (fighters and shuttlecraft) carried by the mothership could be equipped with a simpler, cheaper, smaller, and/or lighter cloaking system that is far more detectable than the normal one.

Alternately, the aliens have no way of getting around their cloak either, and have to turn off the cloak to dock their parasites safely.


So, there's a problem with cloaked ships, which is that if you can't see them, they can't see you either. If the cloaking device is for, say visible light, then the visible light would bend around them and they wouldn't be able to see out. If the cloaking device covers radio waves and radar, then they wouldn't be able to use those either.

So to flip the question on it's head - how are your aliens gettings information from/to the outside world? Whatever method it is, it's distrubing the outside world and can be detected.

If they are happily sitting in a dark silent void then you are back into 'crash something into it' territory.


Two ways: emission or occlusion.

The ship almost certainly has an energy and radiation signature that is different from the surrounding environment. Hotter, colder, more magnetic, and so on. Sensitive detectors are going to spot those differences, especially if the home team suspects the presence of the stealth ship.

Or the ship will block the background behind it. Since the ship is in the air, there are many thousands of different points of view. It is unlikely that the ship can perfectly replicate the background from each of those points of view. If it can, then the home team can kiss their hindquarters good bye; but your job is to make sure that there is some small flaw that the home team can (eventually) exploit.

Stealth works best if the intent is to not call attention to the presence of the stealth-ed ship. The premise is that the ship could be anywhere (including not here at all) and the home team has too many places to look to be able to apply the analysis to pinpoint it.


They're cloaked to the Earth - ok. But are they cloaked all the way around? If only the front is cloaked, a ship returning from the moon might spot them, etc.


Detecting the undetectable, undetectably

Detection is binary measurement. All objects that can be measured are measurable. If an object is not measurable you cannot measure it. You cannot detect that which is not measurable.

If the problem is only that your measurement device is insufficiently sensitive to define a reliable threshold and to turn it into detector, then there is a solution.

Prevent fueling the arms race

Why? You're on the loosing side of it. You are unable to measure the mothership.

How? Covertly increasing measurement sensitivity, by first covertly implementing procedures of measurement detection evasion.

That's a very complex logical problem.

I made a rough and partial draft, it's nothing much just some procedures and provisional bounds.

Covert implementation of measurement detection evasion

  • move into a known blind spot
  • sharpen definitions of your known and potential measurement capabilities
  • sharpen definitions of mothership's known and potential measurement capabilities - must be considered a strict superset of our potential measurement capabilities, except where blind spots are known
  • sharpen definitions of what constitute blind spots
  • sharpen measurement detection evasion procedures
  • improve your measurement sensitivity by covertly replacing sensors with covertly developed higher-quality material - which is only possible if the source materials where first stored in blind spots

This is important, because direct measurements can be increased in sensitivity through scientific invention. Indirect measurements can be increased in sensitivity through artistic invention, but that cannot be implemented by a people, a government or a military, only by the people that constitute them. So we're left with increasing direct measurement sensitivity.

Covertly increasing direct measurement sensitivity

As you may have notice, I've mentioned no increase in the detection range. That's because an increase in the detection range requires moving into areas not covered by current means of measurement detection evasion.

However, direct (e.g. scientific) measurements covered by current management detection evasion, may be increased in sensitivity if you have blacksites to do the development of better sensors.

However, things which were previously measured indirectly can become measured directly if the indirect measurements become included in the measurement detection evasion procedures, which can in turn only be done after enumerating them without revealing information about current measurement detection evasion measures.

This means, that the enumeration must be done using

There are already be de-facto indirect measurement detection evasion procedures in place. I can't elaborate on this further.

General measurement evasion procedures

They must be enumerated. Enumerating them must be done using means which do not increase their risks of detection.

And there is no possibility of covertly implementing general measurement evasion procedures covering de-facto measurement detection evasion procedures without enumerating them first.

There is also no reason to implement general measurement evasion, unless it is implemented in a way completely uncorrelated (both positively and negatively) to the current set of measurement detection evasion procedures and the enumerated de-facto indirect measurement detection evasion measures.

I'm calling it quits, this is too large a problem for Worldbuilding Stackexchange.

Your question is like "how do I win the cold war", heck. Who won that war, anyway?

I don't know. Glonast and perestroika is what the Russians did?

The "Go Stupid" solution

We assume the mothership can't adapt its cloaking procedures and has to be measured for reasons other than military, or is at least in some way inert and unresponsive to active attempts at measurement.

Then it becomes very simple - terminally increase the cost of avoiding detection. Create probes that measure the spacetime around them and shoot them through or around the space where the mothership can be, put a measurement probe behind it and shoot lasers at that and measure changes in the light.

Even just probes that measure the electric charge and chemical structures of particles passing around or through it.

Attempt to detonate a hydrogen bomb at its center and measure both the fallout and the light emitted. Detonate a hydrogen bomb behind the vessel and measure it from 10,000 Earth observatories.

I don't know. If it's a completely inert mothership it's easy work.


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